The present invention relates to a shift control system for an automatic transmission, which is enabled at the time of a shift to drop the engine torque and accordingly the shift shocks by throttling the throttle valve of the engine.
There is already known widely in the prior art a shift control system for a vehicular automatic transmission, which is equipped with a gear shift assembly and a plurality of frictional engagement means so that the engagement of the frictional engagement means is selectively switched by actuating a hydraulic control system to achieve any of a plurality of speed changes.
There are also already proposed a variety of unitary control systems to be used with the automatic transmission and the engine, for controlling the engine torque to be reduced at the time of a shift. If the engine torque is reduced at the shift time, the energy absorption in each of the revolving members of the automatic transmission or the frictional engagement means for braking the members can be suppressed to complete the shift for a short time period and with small shift shocks.
Generally speaking, in case the engine torque is to be changed at the shift time, it is preferable to drop the engine torque while the revolving members of the automatic transmission have their rotational speed changing for the shift, if the shift is a power-on upshift (i.e., an upshift in the state having the accelerator depressed). Here, this time period will be called the "inertia phase". This is because the shift shocks might be increased if the changing timing of the engine torque extends the inertia phase. Since the detection of the inertia phase is not accurate if it is carried out by a timer, it is usually performed directly from a detection of the change in the revolutions of the revolving members.
Therefore, the engine torque control means is required to have an excellent responsiveness. The reason for this requirement will be described in the following. Generally speaking, the energy absorptivity in the frictional engagement means is maximized in the vicinity of the initial stage of the inertia phase. If a torque-down is instructed simultaneously as the inertia phase is detected, for example, it is not rapidly carried to a sufficient extent if the responsiveness of the torque-down means is poor. As a result, the reduction in the thermal load in the frictional engagement means is not fulfilled.
Incidentally, the torque-down means is exemplified by a technology for throttling an intake air flow rate, as disclosed in Japanese patent Laid-Open No. 58-174749. According to this technology, the torque-down can be performed frequently or for a long time. Another high merit obtainable is that a high torque reduction can be achieved. However, the means for driving the valve for throttling the intake air flow is restricted in capacity to have a poor responsiveness so that it is not actually adopted in the existing practice.
On the contrary, there is conventionally used a technology for controlling the torque change having excellent responsiveness, as exemplified by that (i.e., the ignition timing delay technology), which is used for delaying the ignition timing of the engine. However, this method is frequently followed by a gradual rise of the temperature of the exhaust system so that it cannot be used for a long-time or for frequent torque-down. Another problem is that a sufficiently high torque-down cannot be obtained.